In a pulse output stage, the solenoid valve is activated by a voltage pulse, which sets a specific output volume of the pumping device. During the triggering, a magnetic force builds up, which acts against a spring force. As soon as the magnetic force is greater than the spring force, the magnet armature moves and the valve is switched.
Following the trigger pulse, the magnetic force must be extinguished again so that the magnet armature returns to the starting position. Only when the magnetic force is smaller than the spring force can the magnet armature be released. In the ideal case, the current returns to zero during the extinguishing process before the next trigger pulse occurs. This time interval between the complete extinction and the next trigger pulse has the advantage that the starting conditions for a switching pulse are always the same. If the current does not return completely to zero in the available time, then the current can build up over several periods. In the worst case this can result in a function of the solenoid valve being no longer ensured due to a residual magnetic force. Furthermore, the valve is thermally more highly stressed by this current buildup and may be destroyed.
Hence, the energy built up in the magnetic field during triggering should necessarily be extinguished completely in the available time. Depending on the extinction voltage, the energy stored in the magnetic field is proportionately converted in the coil or in the suppressor diode. In the case of a 0V extinction voltage, the extinguishing process theoretically takes an infinite amount of time, the energy being completely converted in the coil. At an infinitely high extinction voltage, the current decay time tends toward zero, for the energy is converted completely at the extinguishing element.
In today's pulse output stages, extinction occurs via a free-wheeling diode. Since the extinction voltage of the free-wheeling diode is relatively low (approximately 0.6 to 0.8 V), the current or energy decay occurs accordingly slowly. For this reason, free-wheeling diodes are principally used only for long switching periods.
A method for triggering an electromagnetic load is described in German Patent Application No. DE 42 22 650, in which the electrical voltage induced at shutoff can be reduced via a free-wheeling diode as well as via a so-called rapid-extinction diode. In this case there is a provision to make switching on the free-wheeling diode and the rapid-extinction diode depend on the rotational speed of the internal combustion engine for example. At low rotational speeds, the induced voltage is provided to be reduced exclusively via the free-wheeling diode. With increasing rotational speeds, the duration over which the free-wheeling diode is operated is shortened such that the rapid-extinction diode is switched on earlier and earlier. At high rotational speeds, the voltage extinction occurs only via the rapid-extinction diode.
For higher rotational speeds and/or a greater number of drive cams of the pumping device, the switching periods are shortened and the extinction must accordingly occur rapidly. The more rapid extinction may be achieved for example using an increased extinction voltage via a Zener diode. With a higher extinction voltage, however, the power loss at the Zener diode or at a parallel-connected transistor increases disproportionately. Since engine control units, however, typically lie at the upper limit with respect to power loss, in the case of a rapid extinction, the housing of the control unit must be enlarged so as to be able to dissipate the higher power loss to the surroundings of the control unit. The increase of the size of the control unit results in significant additional costs.